This invention relates antenna systems, and more particularly to a reflector antenna capable of performing diplexing of received and transmitted signals as the signals are radiated through air.
With typical, present day Cassegrain reflector antenna systems, as shown in FIG. 1, a single feedhorn is employed in connection with a main reflector. The single feedhorn is used to receive signals reflected by a subreflector as well as to radiate a transmitted signal toward the subreflector, which is reflected by the subreflector back toward the main reflector, and subsequently reflected by the main reflector back into space.
The use of a single feedhorn required to perform both transmit and receive functions requires the use of a diplexer circuit to be installed in the waveguide or coax transmission line which is coupled to the feedhorn. The diplexer circuit is necessary to isolate the receive and transmit signals. In actual practice, the diplexer must be constructed within the physical dimensions of the waveguide or coax transmission line. With a WR75 waveguide, this requires the diplexer to be constructed within a cross sectional area of about 0.75 inch (19.05 mm) width and 0.375 inch (9.53 mm) height. As will be appreciated, these very small dimensions impose significant design restraints on the diplexer circuit and add to the difficulty in achieving the needed isolation between the transmit and receive signals. Waveguide, coax or microstrip diplexer components also have a higher loss which must be factored into the design of the electronics operating in connection with the antenna system.
Accordingly, there is a need for an antenna system which allows the transmit and receive signals to be separated without the use of a conventional diplexer circuit being placed in the transmission line communicating with the antenna. More specifically, there is a need for a reflector antenna system which is capable of isolating (i.e., diplexing) the transmit and receive signals while these signals are radiating through space. Such an antenna system would alleviate the above-described space constraints associated with present day diplexer circuits required to be disposed in the transmission line. Such a system would also provide less loss and better isolation of the transmit and receive signals, thus potentially resulting in the need for less expensive electronic circuitry being used to operate with the reflector antenna system.
The present invention is directed to a reflector antenna system which is operable to isolate receive and transmit signals being received and transmitted, respectively, by the antenna system while the signals are radiating through free space. The antenna system of the present invention thus eliminates the need for a conventional diplexer circuit to be included in the transmission line coupled to the antenna system.
In one preferred embodiment the antenna system of the present invention comprises a main reflector, a receive feedhorn and a transmit feedhorn spaced apart from the receive feedhorn. A receive bandpass filter is disposed adjacent the receive feedhorn and a transmit bandpass filter is disposed adjacent the transmit feedhorn. The receive bandpass filter allows a propagating electromagnetic wave having a first frequency to pass therethrough, but prevents an electromagnetic wave radiated from the transmit feedhorn, having a second frequency, which is different from the first frequency, from passing therethrough into the receive feedhorn. The hyperboloidal surface of the receive bandpass filter, in effect, scatters or reflects the transmitted electromagnetic wave isotropically to reduce its reflection back toward the transmit feedhorn. Thus, the receive bandpass filter serves to isolate the receive feedhorn from the signal radiated by the transmit feedhorn.
The transmit bandpass filter operates in the opposite fashion. The transmit bandpass filter operates to reflect an electromagnetic wave reflected by the main reflector back toward the receive feedhorn, where the propagating wave is then able to pass through the receive bandpass filter and be received by the receive feedhorn. Thus, the transmit bandpass filter isolates the transmit feedhorn from the receive signals reflected by the reflector.
In one preferred form, the transmit feedhorn, the receive feedhorn, the transmit bandpass filter and the receive bandpass filter are all supported by a tubular support, wherein the tubular support is supported adjacent a reflecting surface of the main reflector. This construction removes the tolerance constraints that are present when designing a diplexer which must fit within a waveguide or coax transmission line, as with previously developed Cassegrain reflector antenna systems. The use of the receive and transmit bandpass filters, which perform a diplexing function on the received and transmitted electromagnetic waves, also improves the isolation between the receive and transmit signals.
In the preferred embodiments the transmit bandpass filter, which functions as the subreflector, comprises a hyperbolic shape. Each of the transmit bandpass and receive bandpass filters include suitable multi-layer, frequency selective surfaces designed to pass therethrough a signal having a desired frequency. Thus, the receive bandpass filter is designed to pass only a signal having a first frequency, while the transmit bandpass filter is designed to pass only a signal having a second frequency which is different than the first frequency. In one preferred form, the receive bandpass filter is designed to pass a received signal having a frequency of about 12.7 GHz. In one preferred from, the transmit bandpass filter is designed to pass a signal radiated from the transmit feedhorn having a frequency of approximately 14.0 GHz.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.